A sonde is an instrument probe used to automatically obtain information (e.g., vibration, pressure, temperature, etc.) about its surroundings (e.g., underground, under water, in the atmosphere, and the like). Seismic sensors are routinely placed within well bores to obtain information regarding the properties, structure, and activity of the earth in the area surrounding the well bore. Seismic sensor sondes may be individual units or multi-sonde tools linked together via, for example, a cable. In order to obtain accurate seismic data, the sensors or sondes are desirably rigidly coupled to the well bore in order to retrieve the seismic data. Often, the outer surface of a well bore is cemented to the surrounding earth, so that by securing the sensors to the well bore, the sensors are in effect coupled to the earth.
The sensors are typically lowered into the well bore many hundreds or even thousands of feet before they are clamped into position. Therefore, it is desirable that the clamp have a low drag as it is lowered into position. Typically, the clamping mechanism of a sensor is in the retracted position while lowering the sensor array/string to the desired position. The clamping mechanism or mechanisms is or are actuated (e.g., extended) to lock the sensor or sensors in place after the desired depth is reached. The activation and deactivation of the clamping mechanism is usually performed remotely by an operator at the surface. Actuation can be electronic, hydraulic, pneumatic, or accomplished using any other suitable mechanism.
Another key parameter of a borehole clamping mechanism is the clamping force versus the total weight of the sensor and housing. It is generally desirable within the industry to have a clamping force-to-weight ratio of 10, whereas clamping forces less than that ratio may not provide an acceptable level of mechanical coupling to the borehole surface. In certain applications, passive bow spring clamps and/or magnetic clamps having a much lower clamping force-to-weight ratio are adequate. These types of clamps are always engaged both during installation and during data collection. As a result of the clamps always being engaged, the total drag force during installation must be overcome by a weight at the bottom of the sensor array or electric tractor; however, there are practical limits to the amount of weight that can be added to the bottom of the sensor array, and tractors require high electrical current, necessitating copper conductors within the entire length of the sensing array and lead cable.
For borehole clamps that are not passive (i.e., they rely on a remotely actuated mechanism to engage and disengage the clamping force), several variations exist. The most widely used clamp employs an integral electric motor and lead screw arrangement to position a clamping mechanism. Advantages of such an arrangement include a high clamping force and a simple design. Disadvantages of such an arrangement include the need for electrical power in the borehole, and design considerations to avoid sparks or electrical discharges down hole. Most such electrically powered systems have a short lifetime in high temperature borehole environments (e.g., above 150° C.).
Clamping mechanisms that rely on hydraulic actuation have also been used. Such mechanisms include expandable bladders or actuator arms actuated by hydraulic pressure. Advantages of such hydraulic mechanisms include no electrical power down hole and a high clamping force. Disadvantages of such hydraulic mechanisms include the hydrostatic effects of the wellbore and the height of fluid in the hydraulic line which could be thousands of feet. Alternatively, a high pressure gas can be used to actuate a down-hole clamp, but the gas pressure must be high enough to overcome the down-hole pressure, which may be tens of thousands of pounds per square inch (psi).
One method that is currently being used to overcome the hydrostatic effects due to the height of the fluid column in a hydraulically actuated system is to use the well bore fluid as the hydraulic fluid. Such systems typically employ a check valve at the bottom of the hydraulic line that allows the well bore fluid to flow into the hydraulic line until the water level in the hydraulic line matches the water level in the well bore. Pneumatic pressure applied from the surface to the hydraulic line closes the check valve and pressurizes the hydraulic line, thus actuating the clamping force. To release the clamping force, a substantially higher and overpressure is applied which releases a blowout plug which relieves the hydraulic pressure. These systems tend to be unreliable, however, and are susceptible to clogging of the valve with wellbore debris.
With the advent of fiber optic down-hole seismic sensors, electronics and electrical power are often unavailable down hole. Thus, it would be desirable to provide a simple, high-performance borehole clamping system that does not require down-hole electronics or electrical power, and that can operate equally well at both high and low pressures and both high and low temperatures. There remains a need in the industry for such a system.